The present invention generally relates to fixtures used for time aligning signal channels of measurement instruments and more particularly to a deskew fixture for time aligning signal channels to the probing tips of measurement probes connected to the signal channels of one or more oscilloscopes.
Oscilloscopes are used for acquiring, processing and displaying electrical signals from a device under test. Oscilloscopes generally have multiple signal channels for acquiring and displaying multiple signals from the device under test. Each signal channel is coupled to the device under test via a measurement probe. In many measurement applications, a user needs to know the timing relationship between signals being measured. Timing delay differences between the signal channels obscure the timing relationship between the signals being measured. It is therefore necessary to time align or deskew the signal channels for accurate timing measurements.
Tektronix, Inc., Beaverton, Oreg., a manufacturer of oscilloscopes, manufactures a deskew fixture under Part No. 067-0405-02 for time aligning the signal channels of their oscilloscopes to the measurement probe tip. The deskew fixture has a housing supporting a circuit board from which extend four sets of square pin connectors. Each set of connectors has a pair of equal height connectors and a pair of unequal height connectors. One of the equal height connectors in each set is coupled to a ground while the other is coupled to an external input signal. The shorter of the unequal height square pins of each set is also coupled to the external signal while the longer square pin is coupled to ground. A BNC type connector is mounted to the housing for coupling the external input signal to the deskew fixture. The center conductor of the BNC connector is coupled by a circuit run formed on the circuit board to a termination jumper. The termination jumper consists of a pair of square pin connectors extending from the circuit board. A termination jumper resistor, such as a 50 ohm resistor, is positioned on the termination jumper. Equal length circuit board runs extend from the termination jumper to the four sets of square pin connectors.
The above described deskew fixture is designed for hands free deskewing of signal channels in which the measurement probes have socket type probing tips or needle type probing tips with socket type probe tip adapters. The equal length square pins are used where the signal probe tip and the ground tip extend the same distance from the measurement probe. The unequal height square pins are used where the probing tip extends from the end of the measurement probe and the ground tip extends from the measurement probe at a distance back of the probing tip. One of the signal channels is designated the reference signal channel and all other channels are time aligned or deskewed to that channel. The external input signal is applied to the deskew fixture and the measurement probe of the reference channel is connected to one of the pairs of the square pin connectors. The measurement probe of the signal channel to be time aligned with the reference channel is connected to one of the other pairs of square pin connectors. Each oscilloscope signal channel has a channel-to-channel deskew range that allows the channels to be time aligned. For example, the TDS7000 Series digital oscilloscopes, manufactured and sold by Tektronix, Inc., have a deskew range of xc2x125 nsec. The TDS7000 Series oscilloscopes have software routines that adjust the acquired digital samples of the input signals from the various signal channels to display the signals in a time aligned manner. Controls on the oscilloscope are used to deskew the signal channels. It is also possible to use variable delay lines in the various signal channels to time align the signal channels.
A drawback to the above described deskew fixture is that measurement probes with needle type probing tips without socket type adapters require the use of probing stations to connect the measurement probes to the deskew fixture. The measurement probes are secured to probing arms of the probing station and the probing arms are positioned to connect the signal probing tips to the square pin connectors having the external input signal. The ground contacts of the measurement probes may be directly connected to the ground square pins if the spacing between the signal probing tips and the ground contacts are the same as the spacing of the square pin connectors. If such is not the case, then ground connection adapters are required to connect the probe ground contacts to the square pin grounds. The need for probing stations is an added cost to the deskew fixture as well as additional time to setup the probing stations to perform the deskew function. A further drawback to the above described deskew fixture is the inability to deskew differential probes that require both a positive and negative signals.
What is needed is a deskew fixture that allows hands free time aligning or deskewing of signal channels of an oscilloscope that are connected to measurement probes having various types of probing tips. The deskew fixture should easily accept measurements probes having needle type probing tips as well as socket type probing tips. The deskew fixture should also easily accept differential measurement probes having either type of probing tip. The deskew fixture should further minimize signal delays propagating through the fixture so that each of the probe tip contacts receive the deskew fixture signal or signals at the same time.
Accordingly, the present invention is a deskew fixture for time aligning signal channels of a measurement instrument that are connected to measurement probes having probing contacts. The deskew fixture has a multilayer circuit board having opposing sides with pairs of mirrored signal launch contacts disposed on the opposing sides of the circuit board. A first pair of the contacts are coupled to electrical ground and a second pair of contacts are connected via equal length, electromagnetically coupled strip lines to a signal source. The deskew fixture has at least first and second probe holders for supporting the measurement probes on the circuit board. One of the probe holders is disposed on one side of the circuit board and the other probe holder is disposed on the other side of the circuit board with the probing contacts of the measurement probes coupled to the signal launch contacts.
The signal launch contacts are preferably square pin contacts extending from the opposing sides of the circuit board with contact pads formed adjacent to and electrically coupled to the square pins contacts. Alternately, the signal launch contacts may include just the square pin contact or the contact pads. One embodiment of the probe holder includes a support member extending from the side of the circuit board for supporting the measurement probe. A biasing spring extends from the side of the circuit board adjacent to the support member for applying a force on the measurement probe to capture the measurement probe between the support member and the spring. The biasing spring includes a cantilever spring member having a slot formed therein defining first and second cantilever springs extending from a spine member. A support member extends in an approximate vertical direction from the spine m ember and has a slot formed therein axially aligned with the cantilever spring member slot defining first and second cantilever spring supports. A detent member extends into the slot. The cantilever spring member is positioned in an elongated slot formed in the circuit board such that the first and second cantilever springs are positioned on the opposing sides of the circuit board and the detent member is received in a detent aperture formed in the circuit board. A second embodiment of the probe holder has a multiple folded spring wire forming an attachment member connected by support members to spring tensioned support arms with one end of the spring tensioned support arms flared outward for receiving the measurement probe. Multiple probe holders may be disposed on each side of the circuit board.
The signal source is preferably mounted on the circuit board with the signal source generating positive and negative signals. The second pair of signal launch contacts receive the positive signal and a third pair of contacts receives the negative signal via equal length, electromagnetically coupled strip lines connected to the signal source. A voltage input connector is mounted on the circuit board that receives voltage power inputs which are coupled to the signal source. The voltage input connector is preferably a USB connector. Alternately, at least a first signal input connector may be mounted on the circuit board having a signal conductor coupled to one of the equal length, electromagnetically coupled strip lines for receiving a signal from the signal source.